For an internal combustion engine operating with a lean air/fuel mixture (lean burn engines), it is desirable to monitor operation thereof to ensure smooth operation. During lean burn operation, the formation of nitrogen oxides (NOx) can be decreased by operating the engine with as lean a mixture as possible. However, there are limits to the extent to which the strength of the air/fuel mixture can be a decreased, since if the mixture is too lean misfiring can occur which can lead to hunting, and in extreme cases, to stalling of the engine.
Efforts are therefore made in lean burn operation to operate the engine with as high an air/fuel ratio as possible (lean burn operation) while avoiding irregular running which may be perceptible to the driver. Thus to reduce nitrogen oxides emissions the engine has to be operated as close as possible to an irregular-running limit. The air/fuel ratio necessary to achieve this object can only be determined in advance very inexactly, since reaching the irregular-running limit depends on many parameters, some of which are subject to long-term or short-term drifting, for example the current engine operating parameters, the geometry of the combustion chambers, the quality of the mixture formation, the condition of the ignition system, etc. If a predetermined air/fuel ratio is used, the engine must be operated using a relatively large margin of safety from an air/fuel ratio corresponding to the irregular-running limit, and this has an unfavourable influence on the nitrogen oxides emissions.
It is basically known to measure the irregular running of an internal combustion engine. To obtain a measured value for the irregular running, it is preferred to evaluate the movement of the crankshaft. Increasing irregularity of running leads to more irregular running of the crankshaft, so that by means of statistical methods (e.g. comparison of the current angular velocity with an average value) a measured value can be determined which represents a measure of the irregular running.